Automatic bias control

ABSTRACT

In an electrostatographic process recorder utilizing coated paper whose resistivity changes as a function of relative humidity, control apparatus for varying the process bias voltage in accordance with the resistivity of the paper in order to maintain a constant image quality.

United States Patent [191 Bresnick AUTOMATIC BIAS CONTROL [75] inventor: Herbert L. Bresnick, Rochester,

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: Nov. 20, 1972 [21] Appl. No.: 307,911

52 user 307/149, 317/262 R [51] Int. Cl G03g 15/00 [58] Field of Search 317/262 R, 262 A; 307/149 [56] References Cited UNITED STATES PATENTS 3,463,934 8/1969 Nobis 307/149 33 we; m

[ June 11, 1974 3,586,908 6/1971 Vosteen 317/262 A Primary Examiner-L. T. Hix

[ 5 7] ABSTRACT 7 Claims, 5 Drawing Figures VO/Mge Divider 547,2

PATENTE JUN 1 3 m4 SHEET 3 BF 4 Bias Vol/age Voltage Divider 1 AUTOMATIC BIAS CONTROL BACKGROUND OF THE INVENTION Thisinvention relates to electrostatographic process recording apparatus and, more particularly, to apparatus for maintaining image quality constant as environmental conditions vary.

ln electrostatographic process recording apparatus which utilize coated papers, it often becomes necessary to alter the development process by typically varying the bias voltage of the developing member in order to maintain the image quality constant as environmental conditions vary. This requirement is due mainly to the variation in resistivity of the base paper, which is purposely made conductive by the use of salts or soluble conductive polymers. The environmental condition which most affects the resistivity of the paper is the relative humidity in the ambient atmosphere. It would therefore be desirable to keep the resistivity constant without affecting either the cost or the appearance of the printed product.

SUMMARY OF THE INVENTION In accordance with principles illustrative of this invention, circuitry is advantageously provided for controlling the bias voltage of a developing member in order to achieve the effect of a constant resistivity paper. The resistivity of the paper is sensed and, as humidity affects the resistivity, control signals are generated which adjust the bias voltage level which is applied to the developing member.

DESCRIPTION OF THE DRAWING The foregoing will become more readily apparent upon reading the following description in conjunction with the drawing in which:

FIG. 1 schematically depicts a typical paper path and the supply rollers therefor,

FIG. 2(a) graphically depicts the surface resistivity as a function of the relative humidity for a typical paper,

FIG. 2(b) graphically depicts the optimum process bias voltage as a function of the relative humidity,

FIG. 3 depicts a block diagram of an illustrative control apparatus responsive to the resistivity of the paper for controlling the bias voltage level, and

FIG. 4 is a more detailed schematic diagram of an illustrative circuit embodiment of the block diagram of FIG. 3.

GENERAL DESCRIPTION Referring now to FIG. I, depicted therein is a schematic representation of the paper path in an electrostatographic process recorder. The paper travels from supply spool l to take-up spool 2 past imaging station 6 and development station 3. The dotted lines represent the resistance R,I of the paper which may be sensed across conducting rollers 4 and which are used in transporting the paper.

FIG. 2(a) graphically depicts the surface resistivity of a typical coated paper as a function of relative humidity. Such a paper may be, for example, coated with conductive polymer 261 manufactured by Calgon Corp. in the amount of two pounds per ream (1,300 square feet). FIG. 2(b) graphically depicts the desired process bias voltage as a function of the relative humidity for 2 the paper whose characteristics are depicted in FIG. 2(a).

FIG. 3 depicts a block diagram of an illustrative control system in accordance with the principles of this invention. The resistance R, of the paper is sensed across rollers 4 and 5 connected to a bridge circuit 31. An output from bridge circuit 31 is fed to a preamplifier 33 whose output is one input to an operational amplifier 35. The output of operational amplifier 35 is then transmitted to voltage divider 37 where it acts as a control signal to control the generation of the required bias voltage. Voltage divider 37 sends a feedback signal to operational amplifier 35 to complete the control loop.

DETAILED DESCRIPTION Referring now to FIG. 4, depicted therein is a detailed schematic diagram of an illustrative circuit embodiment of the block diagram depicted in FIG. 3. Resistors 41, 42, 43 and the paper resistance R,, form a wheatstone bridge, to which is applied a supply voltage V Resistors 41, 42 and 43 and voltage V l are selected so that the bridge output voltage V on line 45 is always positive over the range of paper resistivity as depicted in FIG. 2(a), and are also chosen so that V decreases as R,, decreases, a monotonic function. Line 45 is connected to field effect transistor 47, connected in a follower configuration as a preamplifier to operational amplifier 35 for impedance matching purposes.

Resistors 50, 52 and transistor 55 comprise voltage divider 37 (FIG. 3). The feedback loop from voltage divider 37 to operational amplifier 35 comprises resistors and 61 in a voltage divider configuration. This feedback loop samples the output bias voltage at output terminal 57 and feeds back a small amount of this voltage (V to operational amplifier 35.

Assuming a relative humidity of 50 percent, from FIG. 2(b) the optimum bias voltage is seen to be 300 volts. The values. of resistors 50, 52, 41, 42 and 43 and transistor 55 are selected to provide this voltage output and resistors 60 and 61 are selected to balance operational amplifier 35. Now assume that the relative humidity changes to percent. From FIG. 2(b) it is seen that an optimum bias. voltage of volts is needed. As the relative humidity increases, R, decreases in accordance with FIG. 2(a). Voltages V and V then decrease, unbalancing operational amplifier 35, and causing voltage V, to increase. As voltage V, increases, transistor 55 conducts more heavily, causing the voltage drop across resistor 50 to increase. This causes the output bias voltage to decrease and a proportionate decrease in voltage V is fed back to operational amplifier 35, bringing it back into balance. It is seen that a decrease in the relative humidity would have the opposite effect. The component values required to shape the output voltage to the required function can be easily determined by those skilled in the art.

Accordingly, there has been shown an arrangement for varying bias voltage to achieve the effect of constant surface resistivity of paper as environmental conditions vary. It is understood that the above-described arrangement is merely illustrative of the application of the principles of this invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of this invention.

What is claimed is:

1. In an electrostatographic process recorder, apparatus for controlling development process bias voltage in order to achieve the effect of constant paper resistivity, said apparatus comprising means for sensing the resistivity of the paper,

means connected to said sensing means for supplying a voltage signal as a monotonic function of said resistivity,

output means including a control element for providing said bias voltage, and

control means responsive to said voltage signal and said bias voltage for controlling said control element to vary said bias voltage so as to achieve the effect of constant paper resistivity.

2. The apparatus of claim 1 wherein said supplying means includes a plurality of resistors connected in a Wheatstone bridge configuration, one arm of said bridge being connected to said sensing means.

3. The apparatus of claim 1 wherein said output means comprises a voltage source,

a first resistor connected to said voltage source,

a second resistor connected to a reference level, and

an output terminal connected between said first resistor and said second resistor.

4. The apparatus of claim 3 wherein said control element comprises a transistor having its collector connected to said output terminal and said first resistor and its emitter connected to said second resistor.

5. The apparatus of claim 4 wherein said control means includes an operational amplifier having a first input connected to said supplying means, a second input connected to said output means, and an output connected to the base of said transistor.

6. The apparatus of claim 5 wherein said control means further includes feedback means connected between said output terminal and said second input of said operational amplifier.

7. The apparatus of claim 6 wherein said feedback means comprises a plurality of resistors connected in a voltage divider configuration. 

1. In an electrostatographic process recorder, apparatus for controlling development process bias voltage in order to achieve the effect of constant paper resistivity, said apparatus comprising means for sensing the resistivity of the paper, means connected to said sensing means for supplying a voltage signal as a monotonic function of said resistivity, output means including a control element for providing said bias voltage, and control means responsive to said voltage signal and said bias voltage for controlling said control element to vary said bias voltage so as to achieve the effect of constant paper resistivity.
 2. The apparatus of claim 1 wherein said supplying means includes a plurality of resistors connected in a Wheatstone bridge configuration, one arm of said bridge being connected to saiD sensing means.
 3. The apparatus of claim 1 wherein said output means comprises a voltage source, a first resistor connected to said voltage source, a second resistor connected to a reference level, and an output terminal connected between said first resistor and said second resistor.
 4. The apparatus of claim 3 wherein said control element comprises a transistor having its collector connected to said output terminal and said first resistor and its emitter connected to said second resistor.
 5. The apparatus of claim 4 wherein said control means includes an operational amplifier having a first input connected to said supplying means, a second input connected to said output means, and an output connected to the base of said transistor.
 6. The apparatus of claim 5 wherein said control means further includes feedback means connected between said output terminal and said second input of said operational amplifier.
 7. The apparatus of claim 6 wherein said feedback means comprises a plurality of resistors connected in a voltage divider configuration. 